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I believe, but am not certain, the flapper in the heater boxes had a rubber seal when it came from the factory. If the seal is gone (dried out, cracked, turned to dust) you will be leaking a little very hot air behind the power of the engine fan. At 90 degrees ambient a little leak makes the cab very uncomfortable. I had to replace the wire from the lever to the flapper (a time consuming, tedious operation) to properly adjust, to get a metal-to-metal seal (without the rubber). That stopped the hot air coming out the cabin vent.
You might be able to find the rubber somewhere and then clean, paint & lubricate the flapper box, but it seems to work O.K. metal to metal when properly adjusted.
If you want to isolate the problem, disconnect the heater tube downstream from the flapper and see if the hot air stops coming out of the cabin register. If so, then you know it's the flapper mis-adjustment probably caused by the now missing rubber.
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